This invention relates to jet drop recorders of the general type disclosed in Beam U.S. Pat. No. 3,577,198 and in Mathis U.S. Pat. No. 3,701,998. Such jet drop recorders comprise a series of electrical and fluidic components, including an orifice plate and a charge plate arranged in laminar relationship for generating one or more rows of jets and selectively charging drops originating therefrom. Typically there may be about 250 or more jets formed in each such row, and each jet is stimulated to produce drops at a rate of about 40 kilohertz. All such drops fall through an electrical deflection field, and those which are charged are deflected into a catcher. Uncharged drops deposit on a moving web transported below the recording head.
One of the most difficult problems in the operation of such jet drop recorders is that of achieving satisfactory startup. When ink or other recording liquid is pumped into an initially empty recording head, the jets do not begin flowing freely but rather tend to blob and run together. This wets the top surface of the charge plate, including electrical lead lines plated thereon and ring-type charging electrodes connected thereto. Once such wetting has occurred, it is impossible to perform satisfactory drop charging, and any attempt at normal operation can result in burning out of the lead lines and the charging electrodes. Also liquid on top of the charge plate may effect jet straightness. Thus in early designs it was necessary to follow the startup step with a difficult cleaning step, which was conducted with the jets running. After cleanup the system was ready for recording.
One of the first solutions to the startup problem involved an impulse technique as taught by Martinez et al U.S. Pat. No. 3,661,304. In this startup method a fluid shock wave forces ink through a jet forming orifice at high energy. This method avoids blobbing at the exit side of the orifice and is fairly satisfactory for starting up a single jet. However, the technique has not been found satisfactory for starting up large number of jets.
A startup method which has been found to be satisfactory for starting up a large number of jets is taught in Stoneburner U.S. Pat. No. 3,891,121. In accordance with that invention a source of pressurized air is connected to the recording head, and high pressure air is pumped into the head prior to the admission of ink. Once a flow of pressurized air has been established through the jet forming orifices, then ink is pumped into the recording head. When the ink follows the pressurized air in this manner, the jets start up cleanly without any wetting of charging electrodes. Another somewhat related startup method is taught in Duffield U.S. Pat. No. 3,970,222.
As further taught by the Stoneburner patent, startup may be improved by pumping a flush fluid through the recording head after the flow of pressurized air has commenced and before admission of ink. The flush fluid forms free flowing jets, and after these jets have been established the flush fluid is replaced with the ink.
Still another startup method involves usage of a charge plate having charging electrodes plated inside a series of notches. Because of the configuration of the electrodes it is possible to move the charge plate sidewardly away from the jets during the startup process. This keeps the charging electrodes clean, but does not prevent collection of ink on the lower surface of the orifice plate. Ink so collected may drip onto the charge plate once it has been moved into the operating position.
Other prior art relating to startup includes Stone U.S. Pat. No. 3,346,869, which shows a nozzle cover, and Chen et al U.S. Pat. No. 3,839,721, which shows a vapor chamber to prevent collection of dried ink during shutdown of a recording head.